Despite what a lot of people think, we should note the following about the Apple Battery Case for the iPhone 6/6s and 6+/6+s:
1. Apple went specifically with that peculiar slide-in design for the case because Mophie has multiple patents on its battery case design for cellphones, and Apple wanted to avoid infringing on Mophie's patents.
2. The case is thin at the top and bottom ends specifically to make sure the case does not unduly interfere with the antennas' reception on the top and bottom of the iPhone 6/6+ models.
3. The case has its own external Apple Lightning connector, so when the phone is in the case you can still use your own Lightning cables to sync with a desktop or laptop computer running iTunes or use the Lightning cable to charge both the phone and the battery case if the cable is connected to a 10 W or 12 W Apple charger (the ones that normally come with the iPad, though you get can one at the Apple Store for under US$20).
(By the way, I wonder why didn't Mophie try to build a Juice Pack case with the external Lightning connector through the Apple Made for iPhone (MFI) certification program? Sure, it may have added around $15 to the retail cost of the Juice Pack case, but then you don't need to fumble around for a Micro USB cable to charge the case and the iPhone in the case can still be synced to a desktop or laptop computer.)